Yes, MiraLAX does have a shelf-life and expiration.

According to the frequently asked questions on MiraLAX.com, "It is not recommended to use MiraLAX beyond the labeled expiration date.”

However, I do think some factors influencing its expected shelf-life or integrity may also depend upon how it was stored as well as how long it has been since the product expired. If it hasn't been too long (e.g. a few months), my personal opinion is that it's probably fine, as expiration dates themselves can be arbitrary numbers, but you're still probably better off getting a new batch if you want to play it safe.

Regarding dosing instructions, the usual adult dosing for MiraLAX is one cap full (approx. 17g) taken once daily or as needed. Apparently the measuring line on the bottle cap itself is supposed to be used as a guideline for taking a single dose (which also measures out to about 1 large tbsp). Ref: [1]

Additional product information from DailyMed: [2]

I hope this info helps!